A document reading apparatus having a document conveying device is known. This document reading apparatus includes a document reading unit for reading information on a document that is conveyed. The document conveying device includes two conveyance rollers. One conveyance roller is disposed on a downstream side in a conveying direction when viewed from the document reading unit and the other conveyance roller is disposed on an upstream side in the conveying direction when viewed from the document reading unit.
Each conveyance roller is composed of a pair of rollers including an upper roller and a lower roller. The document is nipped between the opposing contact portion (nip portion) of the upper and lower rollers and conveyed by rotation of the conveyance roller.
A phenomenon in which a conveyance speed of the document changes when the end edge of the document passes through the nip portion of the conveyance roller located on the upstream side is known. To avoid this phenomenon, a method for preventing a change in speed and a method for reducing the amount of change in speed are proposed.
These methods are disclosed in patent documents, for example, the Japanese Patent Application Laid-Open No. 2005-289544, the Japanese Patent Application Laid-Open No. 2004-051365 and the Japanese Patent Application Laid-Open No. 2002-196791. In the methods disclosed in the above patent documents, a nip pressure is released or reduced before the end edge of the document passes through the nip portion.
In this method, a control circuit and an actuator are needed in order to release or reduce the nip pressure at an appropriate timing. When the control circuit and the actuator are used, the cost of the device increases and additionally, the energy consumption increases. Accordingly, it is desirable to employ a method which is simple and of low energy consumption.
The Japanese Patent Application Laid-Open No. 2004-037965 propose a document conveying device in which pressure release means is disposed and the nip pressure is released after a predetermined time has elapsed from nipping the document by the conveyance roller when the end edge of the document passes through the nip portion of the conveyance roller disposed on the upstream side of a document reading position.
However, in the structure of the invention disclosed in the Japanese Patent Application Laid-Open No. 2004-037965, means for measuring the predetermined time and means for releasing the nip pressure are additionally needed when the pressure release means is used. This causes an increase in the cost of the document conveying device.
FIG. 9A is a graph of a pushing force ratio that is a value obtained by normalizing a force to push a document by a nip pressure for each sheet thickness versus thickness of a sheet document. FIG. 9B is a graph of a calculated value of a pushing distance that is a distance over which a document is pushed versus thickness of a sheet document.
A horizontal axis in FIG. 9A and FIG. 9B represents a thickness of a sheet. The sheet thickness shown in FIG. 9A and FIG. 9B is a range of a thickness of a sheet conveyed by a general document reading apparatus.
In the calculation of the pushing force and the pushing distance, a usual value of the general document reading apparatus is used for a diameter of the conveyance roller or the like and the nip pressure is set to “1”. Namely, it is observed from FIG. 9A and FIG. 9B that when the document having the sheet thickness of about 0.18 mm is conveyed, the document is pushed about 1.2 mm when the end edge of the document passes through the nip portion and a peak value of the pushing force ratio is a bit more than 30% of the nip pressure.
Namely, in an example shown in FIG. 9A and FIG. 9B, when the document is pushed over the pushing distance, the pushing force is continuously applied to the document and the conveyance speed continuously increases.
The change in the conveyance speed can be suppressed by controlling the rotation of the conveyance roller shaft. However, this method is not efficient for the following reason. When a brake is employed as a mechanism to control the rotation of the conveyance roller shaft, a size of the brake exceeds the practical range. This causes an increase in the cost of the document conveying device.